Ion trap mass spectrometers, or quadrupole ion stores, have been known for many years. Ions are formed and contained within a physical structure by means of electrostatic fields, such as DC and AC, e.g., radiofrequency (RF), and a combination thereof. In general, a quadrupole electric field provides an ion storage region by the use of a hyperbolic electrode structure or other electrode structure that provides an equivalent quadrupole trapping field.
The storage of ions in an ion trap is achieved by operating trap electrodes with a time-varying trapping electric field having a trapping amplitude and a trapping frequency, a DC voltage, and device sizes such that ions having mass-to-charge ratios within a finite and useful range are stably trapped inside the device. The aforementioned parameters are sometimes referred to as trapping parameters and from these one can determine the range of mass-to-charge ratios that will permit stable trajectories and the successful trapping of ions.
For stably trapped ions, ion motion may be described as an oscillation containing innumerable frequency components, the first component (or secular frequency) being the most important and of the lowest frequency, and each higher frequency component contributing less than its predecessor. For a given set of trapping parameters, trapped ions of a particular mass-to-charge ratio will oscillate with a distinct secular frequency that can be determined from the trapping parameters by calculation.
In an early method of ion trap operation, the “mass-selective instability mode” (described in U.S. Pat. No. 4,540,884), a mass spectrum is recorded by scanning the trapping amplitude whereby ions of successively increasing m/z are caused to adopt unstable trajectories and to exit the ion trap, where they are detected by an externally mounted detector. The presence of a light buffer gas such as helium at a pressure of approximately 1.3×10−1 Pa was also shown to enhance sensitivity and resolution in this mode of operation.
Although the mass-selective instability mode of operation was successful, another method of operation, the “mass-selective instability mode with resonance ejection” (described in U.S. Pat. No. 4,736,101) proved to have certain advantages, such as the ability to record mass spectra containing a greater range of abundances of the trapped ions. A supplementary (excitation) field is applied across the end cap electrodes and the trapping amplitude is scanned to bring ions of successively increasing m/z into resonance with the excitation field, whereby the ions are ejected and detected to provide a mass spectrum.
The mass resolution of the ion trap mass spectrometer can be improved by scanning in such a way that ions are brought into resonance, ejected, and detected is at a rate such that the time interval between the ejection of successive m/z values is large (e.g., at least 200 times the period of the excitation (resonance) frequency). This technique has allowed the ion trap to be used to distinguish isobaric ions and to resolve peaks due to multiply charged ions of successive masses. Although the resonance ejection enhancement of the mass selective instability scan allows an increased mass range and mass resolution, the scan rate is slow and resolving fractional difference in masses is difficult.
U.S. Pat. No. 5,347,127 to Franzen purported to improve the scan rate for devices having a non-linear field resonance, but in effect only ejected ions at 1 Th intervals. The trapping frequency and the excitation frequency were made to be integer fractions of each other. The scan rate was chosen such that a specified integer number of cycles (e.g., 7) of the excitation frequency were used to analyze each mass change of 1 Th. But, a resolution of a fraction of a Th was not achieved or even attempted.
It is therefore desirable to have improved methods of fast scan rates while maintaining high resolution or higher resolution at current scan rates